GRID-CONNECTED SOLAR PV SYSTEMS · GRID-CONNECTED SOLAR PV SYSTEMS (no battery storage) Design...

GRID-CONNECTED SOLAR PV SYSTEMSNO BATTERY STORAGE

Design Guidelines for Accredited Installers

January 2013

(Effective 1 February 2013)

These guidelines have been developed by Clean Energy Council. They represent latest industry best practice for the design and installation

of grid-connected PV systems. © Copyright 2013

While all care has been taken to ensure this guideline is free from omission and error, no responsibility can be taken for the use of this information in the

installation of any grid-connected power system.

GRID-CONNECTED SOLAR PV SYSTEMS (no battery storage)

Design guidelines for accredited installers

Last update: January 2013 1 of 18

CONTENTS

1 GENERAL 2

2 DEFINITIONS 2

3 DESIGNANDINSTALLATIONSTANDARDS 3

4 LICENSING 3

4.1 ExtraLowVoltage(ELV) 3

4.2 LowVoltage(LV) 3

5 DOCUMENTATION 4

6 RESPONSIBILITIESOFSYSTEMDESIGNERS 4

7 SITE-SPECIFICINFORMATION 5

8 ENERGYYIELD 5

8.1 EnergyYieldFormula 6

8.2 Specificenergyyield 10

8.3 Performanceratio 10

9 INVERTERSELECTION 11

9.1 Multipleinverters 11

9.2 Invertersizing 11

9.3 ArrayPeakPower 11

9.4 Arraypeakpower-invertersizing 12

9.5 Arrayde-ratingformula 13

9.6 Matchinginverter/arrayvoltage 14

9.7 Minimumvoltagewindow 15

9.8 Maximumvoltagewindow 17

9.9 InverterDCinputcurrent 18

9.10 Effectsofshadows 18




Theobjectivesoftheseguidelinesareto:

→ improvethesafety,performanceandreliabilityofsolarphotovoltaicpowersystemsinstalled inthefield

→ encourageindustrybestpracticeforalldesignandinstallationworkinvolvingsolar photovoltaicpowersystems

→ provideanetworkofcompetentsolarphotovoltaicpowersystemsdesignersandinstallers

→ toincreasetheuptakeofsolarphotovoltaicpowersystems,bygivingcustomersincreased confidenceinthedesignandinstallationwork.

Theperformanceofareliableinstallationthatfulfilscustomerexpectationsrequiresbothcarefuldesignandcorrectinstallationpractice.Compliancewithrelevantstatehealthandsafetyregulationsisalsonecessary.

ThisdocumentusesthesameterminologyasoutlinedinAS/NZS5033.Twoimportantdefinitionsare:

2.1.1 Wheretheword“shall”isused,thisindicatesthatastatementismandatory.

2.1.2 Wheretheword“should”isused,thisindicatesthatastatementisarecommendation.

1 GENERAL

2 DEFINITIONS

NOTE:Theseguidelinesalonedonotconstituteafullydefinitivesetofrulesand aretobereadinconjunctionwithallrelevantAustralianstandards.Where theseguidelineshaveadditionalrequirementsabovethatstatedinthe Australianstandardsthentheseguidelinesshouldbefollowed.




Accreditedinstallersshallcomplywiththefollowingstandardswhereapplicable:

3.1.1 Thegrid-interactiveinvertershallbetestedinaccordancewiththeAS4777(parts2and3) andlistedontheCleanEnergyCouncil’sapprovedinverterlist.

3.1.2 Thesystemshallcomplywiththerelevantelectricalserviceandinstallationrulesforthe statewherethesystemisinstalled. (NOTE:thelocalelectricitydistributormayhaveadditionalrequirements.)

3.1.3 Theseguidelinessetadditionalrequirementstothestandards.Anaccreditedinstaller orsupervisorisexpectedtofollowtheseguidelinesinadditiontotherequirementswithin therelevantstandards.

3.1.4 Theseguidelineswillbecomemandatoryon1February2013.

3 STANDARDSFORINSTALLATION

AS/NZS3000 WiringRules AS4777.1 Gridconnect-Installation

AS/NZS5033 InstallationofPhotovoltaic(PV)Arrays

AS/NZS1768 LightningProtection

AS/NZS4509.2 Stand-alonePowerSystems-Design

AS/NZS3008 Selectionofcables

AS1170.2 WindLoads

4.1 ExtraLowVoltage(ELV)

4.1.1 Allextralowvoltagewiringshouldbeperformedbya‘competent’person,whichis definedbytheAustralianStandardAS/NZS4509.1stand-alonepowersystemsas: “apersonwhohasacquiredthroughtraining,qualifications,experienceoracombination ofthese,knowledgeandskillenablingthatpersontocorrectlyperformthetaskrequired.”

4.2 LowVoltage(LV)

4.2.1 Alllowvoltagework:>120VDCor>50VACshallbeperformedbyalicensedelectrician.

4.2.2 Alicensedelectricianisrequiredtoberesponsibleforthesafetyofthesystemwiringprior toconnectionofthesystemtothegrid.IfthesystemcontainsELVwiringinstalledby anon-licensedperson,thenaminimumlevelofinspectionbytheelectricianpriorto closingthePVarrayisolatorswouldinclude:anopencircuitvoltagetestoneachPV stringandonthetotalarray.AvisualinspectionofanopenPVjunctionbox(randomly selected)andthemasterarrayjunctionboxisrequiredtocompleteajob.

Theseinspections/checksshallconfirm:

→ thearrayvoltagesareasdesignedandspecified

→theappropriatecables(CSAandinsulation),junctionfittingsand enclosureshavebeenused.

Boththenon-electricianELVinstaller,aswellasthelicensedelectrician, areexpectedtocarryoutthechecksontheELVwiring.

4 LICENSING




Thedesignerisrequiredtoprovidethefollowingdocumentationtotheinstaller:

→ Alistofequipmentsupplied.

→ Alistofactionstobetakenintheeventofanearthfaultalarm.

→ Theshutdownandisolationprocedureforemergencyandmaintenance.

→ AbasicconnectiondiagramthatincludestheelectricalratingsofthePVarray,andtheratingsof

allovercurrentdevicesandswitchesasinstalled.

→ Site-specificsystemperformanceestimate.

→ Recommendedmaintenanceforthesystem.

→ Maintenanceprocedureandtimetable.

Systemdesignersmustcomplywiththefollowingresponsibilities.

→ Providefullspecificationsofthesystemincludingquantity,makeandmodelnumberofthesolar modulesandinverter.

→ Provideasitespecificfullsystemdesignincludingallshadingissues,orientationandtilt,alongwith thesystem’ssite-specificenergyyield,includingaveragedailyperformanceestimateinkWhfor eachmonthofsolargeneration.

→ Ensurearraydesignwillfitonavailableroofspace.

→ EnsurearraymountingframeinstallationwillcomplywithAS1170.2.

→ Ensurearrayconfigurationiscompatiblewiththeinverterspecification.

→ Ensureallequipmentisfitforpurposeandcorrectlyrated.

→ Obtainwarrantyinformationonallequipment.

→ Whendesigningagridconnectbatterybackupsystemthedesignshallbeperformedbyaperson(s) withCECgridconnecteddesignaccreditationandCECstand-alonedesignaccreditation.

5 DOCUMENTATION

6 RESPONSIBILITIES OFSYSTEMDESIGNERS




Todesignasystemthefollowingsite-specificinformationisrequiredasaminimum:

→ occupationalsafetyrisksofthesite(scaffolding,fallprotect,elevatedworkplatformrequired)

→ whethertheroofissuitableformountingthearray

→ solaraccessforthesite

→ whetheranyshadingwilloccuranditsestimatedeffectonperformance

→ orientationandtiltangleoftheroof

→ wheretheinverterwillbelocated

→ locationofACswitchboards

→ whetheranyswitchboardormeteringalterationsarerequired.

Australiansystemsaretypicallysoldbasedonpriceorthesizethatwillfitontotheavailableroofspace.Oncethesize,inkWp,isselectedthenthedesignershalldeterminethesystem’senergyoutputoveroneyear(knownastheenergyyield).

7 SITE-SPECIFICINFORMATION

8 ENERGYYIELD

Therearemanycommercialtoolsavailabletoassistincalculatingenergyyield,forexamplePV-GC,SunEye,PVSyst,SolarPathfinder,etc.Someofthesemakeanallowanceforshading.

Itisrecommendedtouseoneofthesetoolsonthesitevisittoprovideaccurateestimatesofenergyyield.




8 ENERGYYIELD

NOTE:Theaboveformulaforenergyyieldcouldberearrangedtodetermine thesizeofthearray,ifthesystemistobedesignedtoprovidea predeterminedamountofenergyperyear,forexamplewhena customerwantsasystemthatmeetstheirtotalannualenergyusage.

8.1 ENERGYYIELDFORMULA

Theaverageyearlyenergyyieldcanbeestimatedasfollows:

Esys=Parray_STCxƒmanxƒdirtxƒtempxHtiltxη pv_invxη invxη inv_sb

where:

8.1.1 Manufacturer’soutputtolerance

TheoutputofaPVmoduleisspecifiedinwatts,withamanufacturingtoleranceand isbasedonacelltemperatureof25°C(STC). Example: A160Wmodulehasamanufacturer’stoleranceof±3%.The“worstcase”adjusted outputofthePVmoduleistherefore160Wx0.97=155.2W.

8.1.2 De-ratingduetodirt

TheoutputofaPVmodulecanbereducedasaresultofabuild-upofdirtonthesurface ofthemodule.Theactualvalueofthisde-ratingwillbedependentontheactuallocation butinsomecitylocationsthiscouldbehighduetotheamountofpollutionintheair.Ifin doubt,anacceptablede-ratingwouldbe5%.

Example: Thede-ratedmoduleof155.2Wwouldbede-ratedbyafurther5%duetodirt: 155.2Wx0.95=147.4W.

Esys = averageyearlyenergyoutputofthePVarray,inwatt-hours

Parray_STC = ratedoutputpowerofthearrayunderstandardtestconditions,inwatts

ƒman = de-ratingfactorformanufacturingtolerance,dimensionless(refertosection4.2.1)

ƒdirt = de-ratingfactorfordirt,dimensionless(refertosection4.2.2)

ƒtemp = temperaturede-ratingfactor,dimensionless(refertosection4.2.3)

Htilt = Yearly(monthly)irradiationvalue(kWh/m2)fortheselectedsite(allowingfortilt,orientation)

η pv_inv = efficiencyofthesubsystem(cables)betweenthePVarrayandtheinverter

η inv = efficiencyoftheinverterdimensionless

η inv_sb = efficiencyofthesubsystem(cables)betweentheinverterandtheswitchboard.




8.1.3 De-ratingduetotemperature

Asaminimum,inaccordancewithAS4059.2,theaveragetemperatureofthecellwithin thePVmodulecanbeestimatedbythefollowingformula:

Tcell.eff =Ta.day+25oC

where:

Tcell.eff =averagedailyeffectivecelltemperature,indegreesC

Ta.day =daytimeaverageambienttemperature(forthemonthofinterest),indegreesC.

Arrayframesinstand-alonepowersystemsaretypicallytiltedathigheranglesand themoduleshavegoodairflow.Withrooftopgrid-connectedsystems,highertemperatures havebeenobserved.

Forgrid-connectsystemstheeffectivecelltemperatureisdeterminedbythe followingformula:

Tcell.eff =Ta.day+Tr

where:

Tr =effectivetemperatureriseforspecifictypeofinstallation

Ta.day =thedaytimeambienttemperaturein°C.

Solarmoduleseachhavedifferenttemperaturecoefficients.Thesetypicallyrangefrom +0.2%/°Cto-0.5%/°Cdependantonmoduletechnology.(Refertothemanufacturer’s datasheetforexactvalues). Thede-ratingofthearrayduetotemperaturewillbedependentonthetypeofmodule installedandtheaverageambientmaximumtemperatureforthelocation.

8 ENERGYYIELD

Itisrecommendedthatthefollowingtemperaturerise(Tr)appliesfordifferentarrayframes: → paralleltoroof(<150mmstandoff):+35°C → rack-typemount(>150mmstandoff):+30°C → top-of-polemount,freestandingframeandframeonroofwithtiltangle ofabout+20degreestoslopeofroof:+25°C.




Example continued:

Assumetheaverageambienttemperatureis25°C(Ta.day)andthemodule ispolycrystallineandframeisparalleltoroofbutlessthan150mmoffroof.

Theaveragedailyeffectivecelltemperatureis:

Tcell.eff=Ta.day+35=25+35=60°C Intheaboveformulatheabsolutevalueofthetemperaturecoefficient(γ)is applied,thisis-0.5%/°Candcelltemperatureatstandardtestconditionsis

25°C(Tstc)

Thereforetheeffectivede-ratingfactorduetotemperatureis: 1+[-0.5%X(60–25)]=1-17.5%=0.825 Thetemperaturede-ratingbecomes82.5%of147.4Wor121.6W.

8.1.4 Solarirradiationdata

Solarirradiationdataisavailablefromvarioussources,suchastheAustralianSolar RadiationDataHandbookortheMeteorologicalBureau.Theunitsusedareoften MJ/m²/day.ToconverttokWh/m²/day(PSH)divideby3.6.

8 ENERGYYIELD 8.1.3a Temperaturede-ratingformula Thetemperaturede-ratingfactoriscalculatedasfollows:

ƒtemp =1+(γ×(Tcell.eff-Tstc)) where:

ƒtemp =temperaturede-ratingfactor,dimensionless

γ =valueofpowertemperaturecoefficientperdegreesC(seeabove)

Tcell.eff =averagedailycelltemperature,indegreesC

Tstc =celltemperatureatstandardtestconditions,indegreesC.

NOTE:Themanufacturer’sspecifiedvalueofpowertemperaturecoefficient isapplied–includethe-vesignasshownonthedatasheet.Theformula determineswhetherthetemperaturefactorisgreaterorlessthan1due toactualeffectivetemperatureofthecell.

NOTE:Grid-connectedsolarPVsystemsaretypicallymountedontheroofof thehouseorbuilding.Theroofmightnotbefacingtruenorthoratthe optimumtiltangle.ThePSHfigurefortherooforientation(azimuth) andpitch(tiltangle)shallbeusedwhenundertakingthedesign.




8.1.5 Effectoforientationandtilt

Whentheroofisnotorientatedtruenorthand/ornotattheoptimuminclination, theoutputfromthearraywillbelessthanthemaximumpossible.

Tablesareavailabletodownloadfromsolaraccreditation.com.authatcontain thefollowinginformation:

• averagedailytotalirradiationforvariousorientationsandinclinationanglesfor eachmonthoftheyear,representedasapercentageofthetotaldailyirradiation fallingonahorizontalsurface.

Thetablesprovidethedesignerwithinformationontheexpectedaveragedailytotal irradiationforvariousorientationsandinclinationanglesforeachmonthoftheyearas apercentageofthetotaldailyirradiationfallingonahorizontalsurface(whenthearray islocatedflatonahorizontalsurface).Thetablesalsoprovidethedesignerwith informationtocalculatetheenergyyieldoftheaveragedailyperformanceestimate inkWhforeachmonthofsolargeneration.

Tablesareavailabletodownloadfrom solaraccreditation.com.auforthefollowing majorcities:

→ Hobart → Melbourne → Canberra → Sydney → Brisbane → Cairns → Adelaide → AliceSprings → Darwin → Perth

8.1.6 DCcableloss ItisrecommendedthatthemaximumvoltagedropbetweenthePVarrayandtheinverter isnogreaterthan3%. Example continued: Thede-ratedmoduleof121.6Wwouldbede-ratedbyafurther3%duetoDCcableloss: 121.6Wx0.97=118W.

8.1.7 Inverterefficiency Thisisobtainedfromtheinverterspecifications. Example continued (using an inverter efficiency specification of 90%): Thede-ratedmoduleof118Wwouldbede-ratedbyafurther10%duetoinverterefficiency: 118Wx0.90=106.2W.

8.1.8 ACCableloss Itisrecommendedthatthevoltagedropbetweentheinverterandthemainswitchboard notgreaterthan1%

Example continued: Thede-ratedmoduleof106.2Wwouldbede-ratedbyafurther1%duetoACcableloss: 106.2Wx0.99=105.1W.

8 ENERGYYIELD




8 ENERGYYIELD 8.2 SPECIFICENERGYYIELD

ThespecificenergyyieldisexpressedinkWhperkWpandiscalculatedasfollows:

SY=

Esys

Parray_STC

Tocomparetheperformanceofsystemsindifferentregions,shadinglossmustbeeliminated fromthecalculationofenergyyieldforthesitesbeingcompared.

8.3 PERFORMANCERATIO

Theperformanceratio(PR)isusedtoassesstheinstallationquality.Theperformanceratio iscalculatedasfollows:

PR=

Esys

Eideal

where:

Esys =actualyearlyenergyyieldfromthesystem

Eideal =theidealenergyoutputofthearray.

ThePVarraysidealenergyyieldEidealcandeterminedtwoways.

Method1:

Eideal=Parray_STCxHtilt

where:

Htilt =yearlyaveragedailyirradiation,inkWh/m2forthespecifiedtiltangle

Parray_STC=ratedoutputpowerofthearrayunderstandardtestconditions,inwatts

Method2:

Eideal=Hpvx ηpv

where:

Hpv =actualirradiationthatfallsonthearraysurfacearea

ηpv =efficiencyofthePVmodules

and

Hpv=HtiltxApv

where:

Htilt =yearlyaveragedailyirradiation,inkWh/m2forthespecifiedtiltangle

Apv = totalareaofthePVarray.




Theselectionoftheinverterfortheinstallationwilldependon:

→theenergyoutputofthearray

→thematchingoftheallowableinverterstringconfigurationswiththesizeofthearrayinkW andthesizeoftheindividualmoduleswithinthatarray

→whetherthesystemwillhaveonecentralinverterormultiple(smaller)inverters.

9.1 MULTIPLEINVERTERS

9.1.1 Ifthearrayisspreadoveranumberofroovesthathavedifferentorientationsand/ortilt anglesthenthemaximumpowerpointsandoutputcurrentswillvary.Ifeconomic,installing aseparateinverterforeachsectionofthearraywhichhasthesameorientationandangle willmaximisetheoutputthetotalarray. Thiscouldalsobeachievedbyusinganinverterwithmultiplemaximumpowerpoint trackers(MPPTs).

9.1.2 Multipleinvertersallowaportionofthesystemtocontinuetooperateevenif oneinverterfails.

9.1.3 Multipleinvertersallowthesystemtobemodular,sothatincreasingthesysteminvolves addingapredeterminednumberofmoduleswithoneinverter.

9.1.4 Multipleinvertersbetterbalancephasesinaccordancewithlocalutilityrequirements.

Thepotentialdisadvantageofmultipleinvertersisthatingeneral,thecostofanumberofinverters withlowerpowerratingsisgenerallymoreexpensive.

9.2 INVERTERSIZING

Inverterscurrentlyavailablearetypicallyratedfor:

→maximumDCinputpoweri.e.thesizeofthearrayinpeakwatts →maximumDCinputcurrent →maximumspecifiedoutputpoweri.e.theACpowertheycanprovidetothegrid.

9.3 ARRAYPEAKPOWER

Thepeakpowerofthearrayiscalculatedusingthefollowingformula:

ArrayPeakPower=Numberofmodulesinthearrayxtheratedmaximumpower(Pmp) oftheselectedmoduleatSTC.

9 INVERTERSELECTION




SYSTEM 1 SYSTEM 2 SYSTEM 3 SYSTEM 4

a)Proposedarraypeakpower(eg10x200W) 2000 2000 2000 2000

b)75%ofproposedarraypeakpower(Watts) 1500 1500 1500 1500

c)Invertermanufacturers’maximumallowablearraysizespec(Watts)

2100 1900 2100 1900

d)Invertermanufacturers’nominalACpowerrating(Watts)

1700 1700 1200 1200

Ismanufacturers’maxallowablearraysizespecgreaterthanarraypeakpower(c>a)?

YES NO YES NO

IsinverternominalACpowergreaterthan75%ofproposedarraypeakpower(d>b)

YES YES NO NO

Proposedarraypeakpower–Invertersizingacceptable

YES NO NO NO

Worked example:

Usingthedesignguidelinesithasbeendeterminedthatanarrayof6080Wpeak(32X190Wpanels) willmeetthecustomer’sneeds(offsetenergyusage,fitonavailableroofspace,meets customer’sbudget).

Thereforeweneedtocalculatewhat75%ofthearrayis: 6080WX0.75=4560W.

ThismeanstheinverterforthisarraycannothaveanominalACpoweroutputoflessthan 4560Wandthemanufacturerallowsanarrayof6080Wpeaktobeconnectedtoit.

Wherethemaximumallowablearraysizespecificationisnotspecifiedbytheinvertermanufacturer thedesignershallmatchthearraytotheinverterallowingforthede-ratingofthearray (seesection8.1.1to8.1.4).

9.4 ARRAYPEAKPOWER–INVERTERSIZING

InordertofacilitatetheefficientdesignofPVsystemstheinverternominalACpoweroutput cannotbelessthan75%ofthearraypeakpoweranditshallnotbeoutsidetheinverter manufacturer’smaximumallowablearraysizespecifications.

Example of a 2kW array and 4 inverters with different specification

9 INVERTERSELECTION

NOTE:Theinvertermanufacturer’sspecificationshallbeadheredto.




9.5 ARRAYDE-RATINGFORMULA

Inthesectiononde-ratingmoduleperformance,thetypicalPVarrayoutputinwattsisde-rated

dueto:

→manufacturerstoleranceofthemodules →dirtandtemperature.

9.5.1 Inverterwithcrystallinemodules

Basedonfiguresof:

→0.97formanufacture

→0.95fordirt

→0.825fortemperature(basedonambientof35°C).(Refertosection8.1.3)

Thede-ratingofthearrayis: 0.97x0.95x0.825=0.76

Asaresultofthistypeofde-ratingbeingexperiencedinthefield,theinvertercan berated76%ofthepeakpowerofthearray.

9.5.2 Inverterwiththinfilmmodules

Thetemperatureeffectonthinfilmmodulesislessthanthatoncrystallinemodules. Assumingthetemperaturecoefficientisonly0.1%thenthetemperaturede-ratingat ambienttemperatureof35°Cis0.965. Basedonfiguresof:

→0.97formanufacturer →0.95fordirt →0.965fortemperature(basedonambientof35°C. Thede-ratingofthearrayis:0.97x0.95x0.965=0.889

Asaresultofthistypeofde-ratingbeingexperiencedinthefield,theinvertercaneasily berated89%ofthepeakpowerofthearray.

Example: Assumethearraycomprises16ofthe160Wpcrystallinemodulesthenthearraypeak power=16x160=2.56kW.TheinvertershouldhaveamaximumDCinputratingofat leastof2.56kWandanominalACpoweroutputratingof1.92kW(2.56kWX75%).

IfthemanufacturerdoesnotprovideDCinputspecificationsthenfollowingthe aboveguidelines. Thisarraycanbeconnectedtoaninverterwithanoutputratingof: 0.76x2.56kW=1.95kW(forcrystallinemodules)

Ifthinfilmmodulesareusedthentheinvertercouldhaveanoutputratingof: 0.889x2.56kW=2.27kW

9 INVERTERSELECTION




9.6 MATCHINGINVERTER/ARRAYVOLTAGE Theoutputpowerofasolarmoduleisaffectedbythetemperatureofthesolarcells.Incrystalline PVmodulesthiseffectcanbeasmuchas-0.5%forevery1degreevariationintemperature. (NOTE:forotherPVcelltechnologiesthemanufacturersdatamustbeused).

Thetemperaturede-ratingfactorfortheoutputpoweris:

Ftemp=1+[γ x (Tcell_eff-TSTC)] where:

Ftemp=temperaturede-ratingfactor,dimensionless

γ =powertemperatureco-efficientper°C(typically0.005forcrystallinecells)

Tcell_eff=averagedailycelltemperature,in°C(seesectionontemperatureeffectonmodules)

TSTC =celltemperatureatstandardtestconditions,measuredin°C.

Themaximumpowerpointvoltageandopencircuitvoltageareaffectedbytemperatureandthe temperatureco-efficientasa%istypicallyverysimilartothepowercoefficient. The-maximum-effectivecelltemperature°C

Tcell_eff=Tave_amb+Tr where:

Tcell_eff =theeffectivecelltemperaturein°C

Tave_amb=thedaytimeambienttemperaturein°C

Tr =thetemperaturerisedependentonarrayframetypein°C (refertosectionontemperaturede-ratingofsolararrayfortypicalvalues)

Theformulaonpage13canalsobeappliedasthede-ratingfactorforopencircuitvoltageand maximumpowerpointvoltage.Withtheoddexception,grid-interactiveinvertersincludemaximum powerpointtrackers(MPPTs).

Manyoftheinvertersavailablewillhaveavoltageoperatingwindow.Ifthesolararrayvoltage isoutsidethiswindowtheneithertheinverterwillnotoperateortheoutputpowerofthesystemwill begreatlyreduced.Minimumandmaximuminputvoltageswillbespecifiedbythemanufacturer. Themaximumvoltageisthevoltagewhereabovethistheinvertercouldbedamaged.Some inverterswillnominateavoltagewindowwheretheywilloperateandthenamaximumvoltage, higherthanthemaximumoperatingvoltageofthewindow,whichisthevoltagewheretheinverter couldbedamaged.

Forthebestperformanceofthesystemtheoutputvoltageofthesolararrayshouldbematchedto theoperatingvoltagesoftheinverter.Tominimisetheriskofdamagetotheinverter,themaximum voltageoftheinvertershallneverbereached.

Asstatedearlier,theoutputvoltageofamoduleiseffectedbycelltemperaturechangesinasimilar wayastheoutputpower.

ThePVmodulemanufacturerswillprovideavoltagetemperatureco-efficient.Itisgenerally specifiedinV/°C(ormV/°C)butitcanbeexpressedasa%/°C.

Todesignsystemswheretheoutputvoltagesofthearraydonotfalloutsidetherangeofthe inverter’sDCoperatingvoltagesandmaximumvoltage(ifdifferent),theminimumandmaximum daytimetemperaturesforthatspecificsitearerequired.

9 INVERTERSELECTION




9.7 MINIMUMVOLTAGEWINDOW Whenthetemperatureisatamaximumthenthemaximumpowerpointvoltage(Vmp)ofthearray shouldnotfallbelowtheminimumoperatingvoltageoftheinverter.Theactualvoltageattheinput oftheinverterisnotjusttheVmpofthearray,thevoltagedropintheDCcablingmustalsobe includedwhendeterminingtheactualinverterinputvoltage.

Thetemperaturede-ratingfactorcanbeadaptedtodeterminethemaximumpowerpointvoltage ataspecifiedtemperature.

Vmp_cell.eff= Vmp-STC+[γv x (Tcell_eff-TSTC)]

Where:

Vmp_cell.eff =Maximumpowerpointvoltageateffectivecelltemperature,involts

Vmp-STC =MaximumpowerpointvoltageatSTC,involts

γv =Voltagetemperature(Vmp)coefficientinvoltsper°C

Tcell_eff =celltemperatureatspecifiedambienttemperature,measuredin°C

TSTC =celltemperatureatSTC,measuredin°C

Tomaximisetheperformanceofthearray,theminimumarrayvoltageshouldneverfallbelowthe minimumvoltageoperatingwindowoftheinverter.Thenumberofmodulesinthestringshouldbe selectedsothatthemaximumpowervoltageofthearrayforthehighesttemperatureexpectedis abovetheminimumvoltageoperatingwindowoftheinverter.

SincethedaytimeambienttemperatureinsomeareasofAustraliacanreachorexceed35°Citis recommendedthatmaximumeffectivecelltemperatureof70°Cisused.

9 INVERTERSELECTION




9.7 MINIMUMVOLTAGEWINDOW Worked example: Assumethattheminimumvoltagewindowforaninverteris140V.Themoduleselectedhasarated

MPPvoltageof35.4Vandavoltage(Vmp)co-efficientof-0.177V/°C

UsingequationforVmp_cell.effabove,theminimumMPPvoltageatamaximumeffectivecell temperatureof70°C,thetemperaturede-ratingis:

Vmin_mpp= 35.4+[-0.177 x (70-25)] = 27.4V

Ifweassumeamaximumvoltagedropinthecablesof3%thenthevoltageattheinverterforeach modulewouldbe

0.97x27.4=26.6V

ThisistheeffectiveminimumMPPvoltageinputattheinverterforeachmoduleinthearray,

Vmin_mpp_inv

Theminimumnumberofmodulesinthestringcanbedeterminedbythefollowingequation:

Nmin_per_string=

Vinv_min(V) Vmin_mpp_inv(V)

where:

Vinv_min =theminimuminverterinputvoltage

Vmin_mpp_inv =theeffectiveminimumMPPvoltageofamoduleattheinverterat maximumeffectivecelltemperature Theminimumvoltageallowedattheinverter,inthisexample,is140V. TheMPPvoltageriseswithincreasesinirradiance.Sincethearrayistypicallyoperatingwith irradiancelevelslessthan1kW/m²thentheactualMPPvoltagewouldbereduced.

Intheworkedexampleabove,aminimuminvertervoltageof1.1x140V=154Vshouldbeused. Theminimumnumberofmodulesinastringis:

Nmin_per_string=154/26.6=5.8roundedupto6modules.

9 INVERTERSELECTION

NOTE:Theexactvariationisdependentonthequalityofthesolarcellsoit isrecommendedthatasafetymarginof10%isused




9.8 MAXIMUMVOLTAGEWINDOW Atthecoldestdaytimetemperaturetheopencircuitvoltageofthearrayshallneverbegreaterthan themaximumallowedinputvoltagefortheinverter.Theopencircuitvoltage(Voc)isusedbecause thisisgreaterthantheMPPvoltageanditistheappliedvoltagewhenthesystemisfirstconnected (priortotheinverterstartingtooperateandconnectingtothegrid).

Inearlymorning,atfirstlight,thecelltemperaturewillbeveryclosetotheambienttemperature becausethesunhasnothadtimetoheatupthemodule.Therefore,thelowestdaytimetemperature fortheareawherethesystemisinstalledshallbeusedtodeterminethemaximumVoc. Thisisdeterminedbythefollowingequation:

Vmax_oc= Voc_STC+[γv x (Tmin- TSTC)]

where:

Vmax_oc =Opencircuitvoltageatminimumcelltemperature,volts

Voc_STC =OpencircuitvoltageatSTC,volts

γv =voltagetemperatureVocco-efficient,-V/°C

Tmin =expectedmin.dailycelltemperature,°C

TSTC =celltemperatureSTC,°C

InmanyareasofAustralia,theminimumdaytimeambienttemperaturecanbelessthan0°Cwhile thereareareaswhereitneverfallsbelow20°C.

9 INVERTERSELECTION

NOTE:Someinvertersprovideamaximumvoltageforoperationandahigher voltageasthemaximumallowedvoltage.Inthissituation,theMPPvoltage isusedfortheoperationwindowandtheopencircuitvoltageforthe maximumallowedvoltage.

NOTE:Itisrecommendedthatthedesignerusetheminimumtemperaturefor theareawherethesystemwillbeinstalled.




9.8 MAXIMUMVOLTAGEWINDOW Intheworkedexample,assumetheminimumeffectivecelltemperatureis0°C:

Voc_STCis43.2V andthemaximumopencircuitvoltage-atminimumeffectivetemperatureis:

Voc_max =43.2+(-0.16×(0-25)) =43.26+(-0.16x-25) =43.2+4 =47.2V

Forourexample,assumingthemaximumvoltageallowedbytheinverteris400V(Vinv_max)

Themaximumnumberofmodulesinthestring,Nmax_per_string,isdeterminedby thefollowingequation:

Nmax_per_string=

Vinv_max(V) Voc_max(V)

=400/47.2=8.47roundeddownto8modules

Intheexamplepresented,thePVstringmustconsistofbetween6-8modulesonly. Intheworkedexample,forsizingtheinverter16moduleswererequired.

Thereforewecouldhavetwoparallelstringsof8modules.

9.9 INVERTERDCINPUTCURRENT

EnsurethatthetotalshortcircuitcurrentofthearraydoesnotexceedthemaxDCinputcurrent specificationoftheinverter.

9.10 EFFECTSOFSHADOWS

Intownsandcitieswheregrid-connectedPVsystemswillbedominant,theroofofthehouseor buildingwillnotalwaysbefreeofshadowsduringpartsoftheday.Careshouldbetakenwhen selectingthenumberofmodulesinastringbecauseshadingcouldresultinthemaximumpower pointvoltageathightemperaturesbeingbelowtheminimumoperatingvoltageoftheinverter.

9 INVERTERSELECTION

Date post:	28-May-2020
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